Magnetoresistive read transducer

Abstract: A transducer assembly (10) has passive end regions (26) separated by a central active region (20). Layers of a first biasing material (14) and a nonmagnetic decoupling spacer material (16) are deposited on a substrate (12), then covered by a mask (18) only in the central region. By etching or ion milling, those parts of the layers not covered by the mask are removed to define a transverse biasing means in the central region and define the passive end regions. With the same mask remaining in place, a conductive material (22) and exchange layer (24) comprising a second biasing material are deposited over all regions. The mask is removed to define and provide conductor leads and longitudinal biasing means only in the end regions. MR material is thereafter deposited as a continuous thin film (28) in direct contact with the central region containing the transverse biasing means and in direct contact with the end regions containing the longitudinal biasing means. This fabrication technique has no critical etching steps requiring stopping at or near a particular interface and the MR film provides a continuous platform for carrying current without butted junctions in the current path.

Description: This invention relates to magnetoresistive (MR) read transducers suitable for use in thin film magnetic heads for reading data from magnetic storage devices, and the fabrication thereof. An MR read transducer requires an active region and passive regions. The active region comprises suitable means for transversely biasing the transducer, and each passive region comprises one of a pair of conductor leads and generally suitable means for longitudinally biasing the transducer. The most pertinent prior art known to applicants is US Patent 4,771,349. This patent discloses (in Fig. 2) an MR read transducer having passive end regions separated by a central active region. A continuous thin film of MR material is first deposited over both the central region and end regions. A nonmagnetic spacer layer and a soft magnetic bias layer are deposited and are then patterned to cover only the central active region for imparting a transverse bias to the transducer. A longitudinal magnetic bias layer and conductive leads extend over only the end regions of the transducer. This bias layer creates an interfacial exchange interaction with the MR film that results in an effective bias field that is oriented longitudinally for domain suppression. While this transducer operates satisfactorily, it is relatively difficult to fabricate for the following reason. It is made by applying a mask or "stencil" over the soft magnetic layer and nonmagnetic spacer layer and etching or ion milling to remove these layers outside the mask to define the central region. The problem is that the MR film is generally very thin (e.g., less than 200 Angstroms [20nm]) and it is difficult to stop the etching or milling with a precision that is high with respect to the thickness of the thin MR film. Failure to completely remove the spacer layer in the end regions can render the transducer inoperative; whereas if part of the thin MR film is inadvertently removed, the consequent reduction in its thickness can undesirably impact the sensing characteristics of the transducer. If an attempt is made to restore the MR film to proper thickness by redeposition of MR material, it is difficult to determine how much to deposit because it is difficult to determine in situ how much material was removed. This difficulty in fabrication is also due in part to the fact that the film of MR material is deposited on the substrate as the first step in the fabrication process. US Patent 5,014,147 (in Fig. 4) discloses an MR transducer in which a soft magnetic bias layer is deposited on a substrate, and covered in turn by a nonmagnetic spacer layer, an MR layer, an antiferromagnetic layer and conductor leads. This patent also discloses (in Fig. 6) an MR transducer in which a longitudinal hard bias layer is deposited upon an auxiliary layer and an MR layer is deposited upon the bias layer, followed by a spacer layer, a soft magnetic bias layer and conductor leads. In these embodiments and also in a variation shown in Fig. 8, all the layers extend the full length of the transducer except for the conductor leads. However, the principal disadvantage of this configuration is that the transducer is longitudinally biased over its entire length, which degrades its transverse bias performance and its signal sensitivity. US Patent 5,018,037 discloses an MR read transducer in which an MR layer, a spacer layer of nonmagnetic material and a soft magnetic bias layer are formed as full films and then patterned to cover only a central active region of the transducer. Longitudinal bias layers and conductor leads are then formed in end regions only of the transducer. Note (in Fig. 5) the butted junction in the current path between the MR layer and hard magnetic bias layer. Thus the transducer is composite of three abutting sections which can impact the reliability of the transducer and introduce complications in the magnetic and electrical behavior at the junctions of these sections. Two papers by Tsang, et al. were published in Journal of Applied Physics: (I) in March 1981 at pp. 2471 et seq. and (II) in March 1982 at pages 2605 et seq. Paper (I) discloses use of a NiFe/MnFe/NiFe composite film as an exchange layer in an MR transducer and suggested that such a film would suppress Barkhausen noise. Paper (II) indicated that an exchange structure comprising an MnFe film below (instead of above) a Cu seed layer provided higher exchange magnitude and coercivity at room temperature, higher blocking temperature (T c ), and lower exchange roll off as a function of temperature. Accordingly, the invention provides a magnetoresistive read transducer having a central active region separating two passive end regions, said transducer comprising: a nonconductive substrate, first means including a biasing layer contacting the substrate only in the central region and a nonmagnetic decoupling spacer layer contacting the biasing layer for transversely biasing the transducer; conductive material adjacent the substrate and located only in the end regions to provide two conductor leads; second means contacting the conductor leads for providing exchange layers only in the end regions to longitudinally bias the transducer for domain suppression; and a continuous thin film of magnetoresistive material contacting the exchange layers and decoupling spacer layer to provide a continuous current-carrying platform. The transducer may include a planarizing layer of dielectric material only in the end regions interposed between the substrate and conductive material of the conductor leads to back fill for substrate material excessively removed in the end region. The invention also provides a method of fabricating a magnetoresistive read transducer assembly, said method comprising the steps of: depositing on a substrate a layer of a first biasing material and a nonmagnetic decoupling spacer layer; providing a masking means covering said layers only in a central region; removing those parts of said layers not covered by said masking means to define with the remaining portions of said layers a transverse biasing means in the central region and define at each side of said central region passive end regions devoid of said layers; using said masking means, depositing over said masking means and end regions a conductive material and a second biasing material; removing said masking means to define and provide only in said end regions conductor leads and longitudinal biasing means; and depositing a magnetoresistive material in the form of at least one continuous thin film in direct contact with said central region containing said transverse biasing means and in direct contact with said end regions containing said longitudinal biasing means. In such an MR transducer an MR layer is deposited as a continuous film across the entire width of the transducer during the last step in the fabrication process. This desirably (a) produces an MR transducer with an MR film that provides a continuous current-carrying platform without butted junctions in the current path; and (b) facilitates fabrication by eliminating all etching steps that might require stopping the etching process at a critical interface. The MR read transducer has a transversely biased central active region separated by longitudinally biased passive end regions. The transducer may be fabricated by first depositing a transversely biasing layer on a suitable substrate, then covering it with a nonmagnetic spacer layer. A photoresist mask is then used in a subtractive mode to pattern these layers, such as by etching or ion milling, to cover only the central active region. After the patterning step the same mask is used in an additive mode to deposit only in the end regions conductor leads and then to deposit only in the end regions a longitudinally biasing exchange layer in contact with the conductor leads. After liftoff of the photoresist mask, an MR layer is deposited as a continuous film across the full width of the transducer in direct contact with the exchange layer in the end regions and in direct contact with the nonmagnetic spacer layer in the central active region. Such a fabrication technique has no critical etching steps requiring stopping at or near a particular interface and the MR layer constitutes a continuous platform for carrying current. Since the MR layer is on top of the structure, a capping layer can also be deposited to protect the MR layer during subsequent processing. The transversely biasing layer preferably is a soft magnetic biasing material but may be a conductive shunt layer or hard magnetic biasing material. The exchange layers are preferably multilayered structures, each consisting of a NiFe film that serves as a seed layer under an MnFe film, and another NiFe film that is interposed as a protective layer between the MnFe film and magnetoresistive film. It is possible to replace the NiFe seed layer, if desired, by any appropriate face-centered cubit (or fcc) metal. Alternatively, the longitudinal biasing layer may be a hard magnetic bias material or other antiferromagnetic material. Various embodiments of the invention will now be described in more detail by way of example, with reference to the following drawings: Fig 1A-E depict the sequence of steps in fabricating an MR read transducer in accordance with the invention; Fig. 2 is an end view of an MR read transducer assembly according to one embodiment of the invention; and Fig. 3 depicts a preferred configuration for the exchange layer of the transducer shown in Fig. 2. Figs. 1A-1E show sequentially the steps in the method by which the MR read transducer assembly 10 shown in Fig. 2 is fabricated. As illustrated, the method comprises the steps of depositing on a suitable dielectric substrate 12 a layer 14 of soft magnetic material and then a decoupling spacer layer 16 of nonmagnetic material (Fig. 1A). A suitable photoresist mask 18 is then placed over a portion of spacer layer 16, and the parts of the layers 14, 16 outside the mask are removed, as by etching or ion milling, to define what will become the central active region 20 of the transducer. It is essential that all of the soft magnetic layer 14 not covered by mask 18 be completely removed during this subtractive process. Hence, in practice, it is customary to slightly undercut the substrate 12, resulting in a "pedestal" 12a (Fig. 1B). With mask 18 remaining in place, a layer 22 of conductive material and then an exchange layer 24 of a magnetic bias material are deposited on the substrate 12 and mask (Fig. 1C). An optional planarizing layer 30 of dielective material may be deposited only in the end regions 26 using mask 18, if necessary to backfill for excessive removal of substrate 12 material. After lift off of the resist mask 18, the superposed layers 22, 24 to either side of central region 20 will define passive end regions 26 of the transducer having conductor leads 22a, 22b and exchange layers 24 separated by the central region (Fig. 1D). A layer 28 of MR material is then deposited as a continuous thin film over both the central region 20 and end regions 26, and may be covered if desired by a capping layer 31 of protective material to protect the MR film during subsequent processing (Fig. 1E). Fig. 2 depicts an MR transducer 10 as fabricated by the method described in connection with Fig. 1. As illustrated in Fig. 2, an MR read transducer assembly comprises a bias current source 32 connected between conductor leads 22a, 22b to provide a bias current in central region 20 of the transducer. This current magnetically biases the transducer with a transverse bias only at the central region 20 for producing a linear response mode of the transducer. The exchange layer 24 longitudinally biases only the end regions 26 of the transducer for domain suppression. In the specific embodiment of the transducer 10 depicted in Fig. 2, the material used for soft magnetic film layer 14 was NiFeRh, for the spacer layer 16 was Ta, for the exchange layer 24 was NiFe/MnFe/NiFe, and for the MR layer was NiFe. If desired, the central active region 20 may be transversely biased by substituting for soft magnetic layer 14 either a hard magnetic bias material (not shown), such as a permanent magnet, or a conductive shunt layer (not shown) for shunt biasing the MR material in film 28. The NiFe/MnFe/NiFe structure constituting the exchange layer 24 is illustrated in Fig. 3. This structure consists of a NiFe seed layer 24a in direct contact with the under side of a film 24b of MnFe; and the upper side of the MnFe film contacts a NiFe film 24c that serves as a protective layer and directly contacts the MR layer 28. In this structure the MnFe film serves as an antiferromagnetic layer to ensure longitudinal biasing and the ability of the MR film 28 to sense flux transitions. Increasing the thickness of seed layer 24a improves the efficiency of the exchange by the MnFe film with the upper film 24c and improves the coercivity. The thicknesses of both NiFe films 24a, 24c can be adjusted to meet desired boundary conditions; i.e., the thicknesses of the layers in the central region 20 versus those in the end regions 26. If desired, the exchange layer 24 may be a bias layer comprising a different seed layer (e.g. any appropriate face-centered-cubic metal), a different antiferromagnetic material (e.g., NiFeMn or NiMn) or a bias layer of hard magnetic material (e.g., a permanent magnet) deposited over a suitable nonmagnetic underlayer. This underlayer may be either separately deposited or included as part of the conductive material 22 used for the conductor leads 22a, 22b; e.g., Ta separately deposited or Ta included in a trilayer Ta/Au/Ta conductor lead material. Thus the fabrication method involves no critical etching or additive steps and provides an improved MR read transducer 10 having an MR layer 28 in the form of a continuous thin film that serves as a continuous current-carrying platform without butted junctions in the current path. A capping layer 31 can readily be applied over the MR layer to protect it during subsequent processing. Use of the same mask 18, first in a subtractive mode, then in an additive mode, retains a configuration with aligned boundaries for the central region 20 and end regions 26. Finally, by use of a NiFe/MnFe/NiFe exchange layer 24 with NiFe as the seed layer 24a, higher exchange magnitude and higher coercivity are achieved at room temperature, exchange roll off as a function of temperature is reduced, and operation up to a higher blocking temperature is possible.

Claims: A method of fabricating a magnetoresistive read transducer assembly, said method comprising the steps of: (a) depositing on a substrate (12) a layer of a first biasing material (14) and a nonmagnetic decoupling spacer layer (16); (b) providing a masking means (18) covering said layers only in a central region (20); (c) removing those parts of said layers not covered by said masking means to define with the remaining portions of said layers a transverse biasing means in the central region and define at each side of said central region passive end regions (26) devoid of said layers; (d) using said masking means, depositing over said masking means and end regions a conductive material (22) and a second biasing material (24); (e) removing said masking means to define and provide only in said end regions conductor leads and longitudinal biasing means; and (f) depositing a magnetoresistive material in the form of at least one continuous thin film (28) in direct contact with said central region containing said transverse biasing means and in direct contact with said end regions containing said longitudinal biasing means. The method of claim 1 including, following step (f), the step of: applying a capping layer of protective material (31) over the magnetoresistive material to protect it during subsequent processing. The method of claim 1 or 2, including during step (d) sequentially depositing on the conductive material (22) films (24a, 24b, 24c) of NiFe, MnFe and NiFe; and defining and providing by step (e) an exchange layer in which one of the NiFe films serves as a seed layer under the MnFe film and the other NiFe film is interposed as a protective layer between the MnFe film and magnetoresistive film to form the longitudinal biasing means. The method of claim 1 or 2, including during step (d) sequentially depositing on the conductive material films of a face-centered cubic (fcc) metal, MnFe and NiFe; and defining and providing by step (e) an exchange layer in which the fcc metal serves as a seed layer under the MnFe film and the NiFe film is interposed as a protective layer between the MnFe film and magnetoresistive film to form the longitudinal biasing means. The method of claim 1 or 2, including, during step (d), depositing on the conductive material an antiferromagnetic material as the second biasing material to form the longitudinal biasing means. The method of claim 1 or 2, including, in during step (d), depositing on the conductive material a film of a hard magnetic material as the second biasing material to form the longitudinal biasing means. The method of any preceding claim, including between steps (c) and (d), the step of using said masking means, depositing a planarizing layer of dielectric material only in the end regions to back fill for material excessively removed from the substrate during step (c). A magnetoresistive read transducer having a central active region (20) separating two passive end regions (26), said transducer comprising: a nonconductive substrate (12), first means including a biasing layer (14) contacting the substrate only in the central region and a nonmagnetic decoupling spacer layer (16) contacting the biasing layer for transversely biasing the transducer; conductive material (22) adjacent the substrate and located only in the end regions to provide two conductor leads; second means (24) contacting the conductor leads for providing exchange layers only in the end regions to longitudinally bias the transducer for domain suppression; and a continuous thin film (28) of magnetoresistive material contacting the exchange layers and decoupling spacer layer to provide a continuous current-carrying platform. The transducer of claim 8, wherein said biasing layer is a soft magnetic material. The transducer of claim 8, wherein said biasing layer is a conductive shunt layer for shunt biasing the magnetoresistive material. The transducer of claim 8, wherein said biasing layer is a hard magnetic material. The transducer of any of claims 8 to 11, wherein each exchange layer comprises a NiFe/MnFe/NiFe sandwich structure. The transducer of claim 12, wherein the exchange layers are multilayered structures, each consisting of a NiFe film that serves as a seed layer under an MnFe film, and another NiFe film that is interposed as a protective layer between the MnFe film and magnetoresistive film. The transducer of any of claims 8 to 11, wherein the exchange layers are multilayered structures, each consisting of a face-centered-cubic metal film that serves as a seed layer under an MnFe film, and a NiFe film that is interposed as a protective layer between the MnFe film and magnetoresistive film. The transducer of any of claims 8 to 11, wherein each exchange layer comprises an antiferromagnetic material. The transducer of any of claims 8 to 11, wherein each exchange layer comprises a hard magnetic bias material. The transducer of any of claims 8 to 16, including a planarizing layer of dielectric material only in the end regions interposed between the substrate and conductive material of the conductor leads to back fill for substrate material excessively removed in the end region.

Inventors: KROUNBI MOHAMAD TOWFIK, KUNG KENNETH TING-YUAN, TSANG CHING HWA, KROUNBI, MOHAMAD, KUNG, KENNETH TING-YUAN, TSANG, CHING HWA, KROUNBI, MOHAMAD TOWFIK
Applicants: IBM, INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1994
Main classification: G11B5
IPC codes: G11B5, H01L43